Hybrid rocket propellant using polymeric sponge and liquid oxidizer

ABSTRACT

9. A method for developing thrust in a rocket combustion chamber by combusting a by-propellant which comprises igniting a rubbery sponge having cells and an intercellular matrix of cured solid polymeric substance and propellant fuel in said matrix mixed with the cured polymeric material, said cells containing a liquid oxidizing agent reactive with the fuel and the cured polymeric substance on ignition, said fuel and oxidizing agent composing at least 75 wt.% of the by-propellant.

United States Patent Longwell [451 Nov. 21, 1972 [54] HYBRID ROCKETPROPELLANT USING POLYMERIC SPONGE AND LIQUID OXIDIZER [72] Inventor:John P. Longwell, Scotch Plains,

[73] Assignee: Esso Research and Engineering v Company [22] Filed: June2, 1959 [21] Appl. No.: 817,970

[52] US. Cl ..60/219, 102/99 [51] Int. Cl. ..C06d 5/10 [58] Field ofSearch .....60/35.4, 219; 52/05; 102/98,

[56] References Cited 7 UNITED STATES PATENTS 2,791,883 5/1957 Moore etal7 ..l49/l9 2,802,332 8/1957 Orsino ..l49/l9 X 10/1958 Thomas ..l49/l9OTHER PUBLICATIONS Chem. & Eng. News, May 27, 1957, pp. 18-23 [TPl.T4l8]Zaehringer, Solid Propellant Rockets-Second Stage American Rocket Co.,Box 1112, Wyandotte, Mich., pp. 229-230 L783.3Z3 1958] PrimaryExaminer-Benjamin R. Padgett Attorney-Small, Thomas, Dunham and Marx andR. D. Manahan EXEMPLARY CLAIM 9. A method for developing thrust in arocket combustion chamber by combusting a lay-propellant which comprisesigniting a rubbery sponge having cells and an intercellular matrix ofcured solid polymeric substance and propellant fuel in said matrix mixedwith the cured polymeric material, said cells containing a liquidoxidizing agent reactive with the fuel and the cured polymeric substanceon ignition, said fuel and oxidizing agent composing at least 75 wt.% ofthe bypropellant.

10 Claims, 1 Drawing Figure PATENTEflnnvzw 1972 John P. Longwell//VVE/VT0/? PATENT ATTORNEY HYBRID ROCKET PROPELLANT USING POLYMERICSPONGE AND LIQUID OXIDIZER The present invention relates to novelsponges which are useful as supports for fuel and/or oxidizing agentsemployed in rocket propellants. Specifically, it concerns the use of acellular polymeric substance as a matrix for materials which are capableof imparting propulsion to rockets.

A rocket or jet propulsion device as used herein is a rigid containerfor matter and energy so arranged that a portion of the matter canabsorb the energy in kinetic form and subsequently eject it in aspecified direction. The type of rocket to which the invention hasparticular application is that which is generally designated as a purerocket, that is to say, a rocket in which the means producing the thrustdoes not make use of the surrounding atmosphere. The rocket is propelledby contacting the fuel or propellant in a combustion chamber with asuitable oxidizing agent which produces burning and therefore causes therelease of energy at a high but controllable rate from the combustionchamber. The solid bi-propellants normally used in rockets consist ofsolid fuel and solid oxidizing agents. While solid oxidizing agents arehighly useful, they are usually less effective than liquid oxidizingagents because the latter in most instances permit attainment of higherspecific impulses.

It is an object of this invention to provide a novel sponge support forliquid and solid propellant components. Still another object is toprepare a hybrid propellant containing a liquid oxidizing agent whichhas the dimensional integrity possessed by solid propellants.

In accordance with the present invention, novel solid polymericsubstances which have a cellular nature are employed to supportsubstances used to propel rockets. Moreover, it has been found thatsponge-like elastomers having open cells will soak up (or otherwiseintroduce) highly active liquid oxidizers into their matrix whichcontains a substantial amount of fuel and thereby form highly suitablehybrid rocket propellants.

1n carrying out the present invention any suitable high molecular weightpolymer, especially one which is rubbery in nature, is used to prepare afoam or sponge which contains open cells, that is to say, cells whichare interconnecting and which are open to the surrounding atmosphere.Known methods for preparing foams or sponges may be employed to make thematrices which hold the liquid oxidizing agent. The polymer should becompounded at an elevated temperature, e.g., l-30 0F., with the rocketfuel and other compounding ingredients used in the rubber an, theblowing agent generally being added last at a lower temperature toprevent a premature blow.

One method for preparing an open cell sponge involves compounding asolid rubbery polymer having a hydrocarbon backbone, such as butylrubber, styrenebutadiene rubber (SBR), polychloroprene rubber,hexafluoropropylene-vinylidine fluoride copolymer and the like with asolid metal fuel, e.g., powdered aluminum, a blowing agent and a curingagent, e.g., sulfur. The compounded polymer is then subjected to anelevated temperature, preferably about 250 to 350F., in a mold or ovenfor up to 2 or 3 hours to form a cured open cell sponge, that is onewhich is capable of absorbing more than about 5 wt. of water. Atypicalrecipe for such a sponge is as follows:

Components Parts By Weight General Preferred Polymer 100 Solid Fuel orFiller 0-400 20-300 Liquid Fuel 0-60 20-60 Blowing Agent 5-30 10-20Activator 2-20 4-12 Accelerator 0-20 1-5 Curing Agent 0-20 1-12 MetalOxide 0-20 1-10 The filler, if used at all, is generally carbon black orclay. If a liquid fuel is employed it is preferred to use a naphthenicor paraffinic hydrocarbon since it will also serve as a processing oil.Solid fuels which may be partly or completely substituted for the fillerin the above recipe include powdered metal and metal-containingsubstances in which the metal is selected from Groups I to IV of thePeriodic Chart of Elements. While fuels such as lithium hydride andtitanium are useful, the preferred fuels are the Group 11 and IIImetals, e.g., aluminum, beryllium, boron, magnesium and their hydrides.The blowing agent is generally a material which will decompose atelevated temperatures to produce oxygenor nitrogen-containing gases thatcause the formation of numerous open cells in the solid polymer.However, other nondecomposing blowing agents such as low boilingparaffinic hydrocarbons in the C to C range may be used. Suitableblowing agents include sodium carbonate, dinitrosopentamethylenetetramine, azoisobutyric nitrile and pentane. It is sometimes desirableto use an activator, such as urea or stearic acid, to initiate theaction of certain blowing agents, e.g., dinitrosopentamethylenetetramine or sodium bicarbonate.

Accelerating agents such as tellurium diethyl dithiocarbamate andtetramethyl thiuram disulfide may be employed to assist the curing agentwhich is usually sulfur when natural rubber, SBR or butyl rubber is thepolymer, zinc oxide when halogenated butyl rubber is used andhexamethylene diamine when copolymers containing large amounts offluorine are used in the preparation of the sponge. It is also sometimesdesirable to compound the polymer with a metal oxide, such as zinc oxideor magnesium oxide, in addition to the curing agent, in order to obtaingood physical properties in the sponge. The compounded polymer should beheated at an elevated temperature which is generally between 200 and400F. for at least 1 minute, e.g., 5-30 minutes. Where the polymer isnatural rubber, butyl rubber, halogenated butyl rubber orstyrene-butadiene rubber, it has been found that heating the compoundedrubber at 250-350F. for about 10-30 minutes in a closed mold produces asponge which has excellent physical characteristics. The aforementionedrubbers generally have viscosity average molecular weights of at least100,000 and sometimes as high as 2,000,000 or more.

Butylrubber, a term well known in the rubber art, e.g., Chapter 24 inSynthetic Rubber" edited by G. Whitby, is a rubbery copolymer comprisinga major proportion of a monoolefin having four to seven carbon atoms anda minor proportion of a rnultiolefin having four to eight carbon atoms.The most commonly employed monoolefin is isobutylene, although othermonoolefins such as 3-methyl-butene-l and 4-methylpentene-l may be used.Suitable multiolefins, which are generally conjugated diolefins, includeisoprene, butadiene-l,3, dimethyl butadiene-l,3, piperylene and thelike. Most of the copolymers contain about 90 to 99.5 wt. isoolefin and0.5 to wt. diolefin, which in most instances is isoprene. Thepolymerization is generally carried out at a low temperature, e.g.,between 50 and 165C., in the presence of a Friedel-Crafts catalyst, suchas aluminum trichloride, dissolved in a lower alkyl halide, such asmethyl chloride, ethyl chloride, etc. Their preparation is fullydescribed in US. Pat. No. 2,356,128. Butyl rubber has a viscosityaverage molecular weight between about 200,000 and 1,500,000 or more anda Wijs iodine number between about 1 and 50.

The physical and chemical properties of butyl rubber may be modified byintroducing a small quantity of halogen into the polymer. In producinghalogenated butyl rubber, unmodified, unvulcanized butyl rubber iscarefully halogenated until it contains about at least 0.5 wt.(preferably at least about 1.0 wt. but not more than about X wt. ofcombined chlorine or 3 X wt. combined bromine wherein:

L mole of the multiolefin in the polymer M, molecular weight of theisoolefin M molecular weight of the multiolefin M atomic weight of thehalogen Restated, there should be at least about 0.5 wt. of combinedhalogen in the polymer but not more than about one atom of chlorine orthree atoms of bromine combined in the polymer per molecule ofmultiolefin present therein; i.e., per double bond in the polymer.

Styrene-butadiene rubber is made by copolymerizing the monomers in anaqueous emulsion at 40 to 120F. in the presence of a peroxide orpersulfate catalyst. The copolymer generally contains about 80 percentbutadiene and percent styrene.

The open cells of the foam or sponge are at least partially filled witha liquid oxidizing agent capable of reacting rapidly with the fuel uponignition to produce low molecular weight gases. The liquid oxidizingagent should not attack the sponge at ambient temperatures. Many of theoxidizing agents useful in conventional bipropellant systems may beemployed. Among the oxidizing agents which may be used in thepropellants are hydrogen peroxide, white fuming nitric acid or redfuming nitric acid, tetranitromethane, bromotrifluoride, etc. Thepreferred oxidizing agents are inorganic and organic nitrogen compounds;the latter compounds generally contain one to four carbon atoms. Thesponge may be immersed in the oxidizing agent or otherwise subjected totreatment so that it soaks up a substantial amount of oxidizing liquid.In order to serve as a high energy propellant, the fully treated spongeor foam should contain about 75 to 95 wt. fuel and oxidizing agent,while the polymer matrix (intercellular substance), exclusive of thefuel but including the polymer and curatives should not comprise morethan wt. and preferably less than 10 wt. of the finished rocketpropellant.

One of the advantages of the present invention is that the fuel may bemaintained separate from the oxidizer until just prior to the use of thepropellant in the rocket. This factor would substantially improve theshelf life of the propellant charges. In other instances the sponge maybe filled with the oxidizer and stored inside the rocket.

It is important in carrying out the present invention to employ aminimum of noncombustible material. It is advantageous to use ahydrocarbon polymer such as butyl rubber or styrene-butadiene rubbersince these materials have some fuel value. They have the added benefitof being readily available at a low cost.

Since some rubbers, such as styrene-butadiene rubber, are prepared byanemulsion technique, the solid foam may be prepared directly from thelatex according to methods well known to those skilled in the rubberart. For instance, a styrene-butadiene rubber latex may be gelled orprecipitated and simultaneously blown with air or oxygen to produce aplurality of interconnecting cells in the solid foam produced from thelatex.

The accompanying drawing is a schematic sectional view of a rocketcontaining a hybrid bi-propellant of the present invention which isshown in place in the rocket in a perspective view.

The bi-propellant in rocket 1 comprises a cylindrical sponge 2 which hasan internal burning surface 3 in the shape of a star. The star-shapedconduit 4 in the center of sponge 2, which is parallel with thelongitudinal axis of said sponge, is formed by the burning surface 3.Conduit 4 may be filled with an igniting substance, e.g., black powder,not shown, which may be ignited by some suitable means, such as anelectric match, also not shown. 0f course the propellant need not becylindrical nor have an internal burning surface. For example, it may becruciform or rod shaped and burn like a cigarette. Thus, any suitableshape or form may be employed. The cells 5 of the sponge 2 are filledwith a liquid or semi-liquid oxidizer and its matrix 6 contains fuel,preferably powdered metal or metal hydride. The outer surface 7 ofsponge 2 may be coated with a layer 8 of nitrocellulose, rubber, resinor other substance which helps to prevent the loss of liquid oxidizer anmay also, where the layer 8 is an adhesive, serve to adhere the outersurface 7 of sponge 2 to the rigid inner wall of the rockets combustionor reaction chamber 9. The combustion gases of the propellant leave thechamber 9 via nozzle 10.

The following examples are given to illustrate some embodiments of theinvention.

EXAMPLE 1 Isobutylene-isoprene butyl rubber having a viscosity averagemolecular weight of 400,000 and a mole unsaturation of 1.5 is compoundedin a Banbury according to the following recipe and cured at 310F. for 30Urea Tellurium Diethyl-Dithiocarbamate 1.25

Tetramethyl Thiuram Disulfide l Sulfur l .25 Zinc Oxide 5 The compoundedrubber which is blown and cured in a 6 X 6 X 0.5 inches mold produces anopen cell sponge having a density of 0.35 grams/cc. The sponge is thenimmersed in 98% H at ambient (75F.) temperature and the sponge afterwithdrawal from the liquid oxidizer contains 50 wt. oxidizing agent.Another portion of the compounded rubber is placed in another mold whichis 6 X /4 X inches and blown and cured as described above. The resultingsponge strand is ignited at the top and permitted to combust down itslongitudinal axis at atmospheric pressure. It is noted that the burningrate is approximately 1 inches per second.

EXAMPLE 2 Viton A, a tradename for a copolymer of hexafluoropropyleneand vinylidene fluoride, is compounded according to the followingrecipe:

Sodium Carbonate The ingredients are mixed on a cold mill, the blowingagent (sodium carbonate) being added last. The compounded copolymer isblown and cured in a 6 X 6 X 1% inches mold at 310F. for 30 minutes toproduce an open cell sponge having a density of 0.4 grams/cc. The spongeis then immersed in tetranitromethane at ambient temperature, and thesponge after being withdrawn from the liquid oxidizer contains 55 wt.tetranitromethane. This fluorocarbon polymer produces a sponge havingexcellent tensile strengths, e.g., 100 psi, and therefore resistscracking under pressure in the rockets burning chamber.

Another portion of the compounded fluorocarbon polymer is cured andblown in a 6 X A X /4 inches mold to produce an open cell sponge strandwhich after immersion in tetranitromethane burns at the rate of onehalfinch per second and produces a green flame. The Viton A used in thisExample has a Mooney viscosity (ML 4/2 1 2F.) of 67 and contains about65 wt. combined fluorine. This solid rubbery copolymer is highlysuitable for the purposes of the present invention because it is highlyresistant to attack by oxidizers. Other fluorocarbon polymers such asthe copolymer of chlorotrifluoroethylene and vinylidene fluoride may beused in place of Viton A. While these fluorocarbon polymers do not havetrue hydrocarbon backbones they are considered to have such a backbonefor the purposes of this invention. If desired, certain liquid oxidizersmay be admixed with a small amount, i.e., less than 1 wt. of athickening or gelling agent such as carboxymethyl cellulose prior tointroducing them into the sponge.

EXAMPLE 3 Natural rubber (smoked sheet) is compounded on a coldlaboratory mill according to the following recipe:

R E C I P E Component Parts By Weight Natural Rubber 100 Aluminum Powder100 Sulfur 5 Tetramethylthiuram Disulfide 2.5 Benzothiazyldisulflde l .5Zinc Oxide l0 Stearic Acid 10 Hydrocarbon Oil 30 The same oil used inExample 1 Portions of the compounded rubber are blown and cured for 30minutes at 310F. in a mold 6 X X /4 inches and the open cell spongestrands produced are immersed in 98% H 0 for 5 to 45 minutes. immersinga strand in H O, for 15 minutes produces a bi-propellant containing 60weight percent liquid oxidizer. When such a strand is ignited, itsburning rate is about 1 inch per second, and white sparks are produced.Instead of merely immersing the sponge in the liquid oxidizing agent, itcan be stored in the rockets burning chamber and evacuated and filledwith oxidizer just prior to use.

The essence of the invention is the use of rubbery sponges in solidrocket propellants to permit the employment of liquid oxidizers whichare generally more effective than solid oxidizers. Any polymer may beused which is capable of forming such a sponge. Resort may be had tovarious modifications and variations of the present invention withoutdeparting from the spirit of the discovery or the scope of the appendedclaims.

What is claimed is:

l. A rocket propellant which comprises a rubbery sponge having cells andan intercellular matrix of a solid rubbery polymeric substance havingcompounded therein a fuel, said polymeric substance being an elastomerblown into a sponge and cured, and said cells being filled with a liquidoxidizing agent reactive with the cured matrix and the fuel compoundedtherein on ignition to generate combustion gas for imparting propulsion,said oxidizing agent and fuel composing at least wt.% of the propellant.

2. A rocket propellant according to claim 1, wherein the liquidoxidizing agent is thickened to a semi-liquid.

3. A rocket propellant according to claim 1, wherein the elastomer is acopolymer of a mono-olefin and a diolefin.

4. A rocket propellant according to claim 1, wherein said fuelcompounded with the rubbery polymeric substance is a powdered metalselected from Groups I to IV of the Periodic Chart of Elements.

5. A rocket propellant according to claim 1, wherein the rubberypolymeric substance is cured butyl rubber.

6. A rocket propellant according to claim 1, in which the rubberypolymeric substance contains halogen.

7. The method for preparing a rocket propellant which comprises admixinga curable rubbery polymer with a powdered fuel, blowing agent, andcuring agent, heating the resulting mixture at elevated temperatureuntil a cured rubbery sponge having cells and an intercellular matrix ofthe cured rubbery polymer and powdered fuel mixed with the polymer inthe matrix is obtained and filling the cells of said sponge with aliquid oxidizing agent reactive with the cured polymer and the fuelcompounded therein on ignition, said oxidizing agent and said fuelcomposing at least 75 wt.% of the propellant.

8. A method for preparing a hybrid rocket propellant which comprisesadmixing 100 parts by weight of a curable rubbery hydrocarbon polymerwith about 20 to 300 parts by weight of fuel, about to 30 parts byweight of a blowing agent and l to parts by weight of a curing agent,heating the resulting mixture to an elevated temperature until themixture forms a cured open cell rubbery sponge and substantially fillingthe cells in said sponge with a liquid oxidizing agent reactive with thefuel, said oxidizing agent and said fuel comprising at least 75 wt.% ofthe propellant.

9. A method for developing thrust in rocket combustion chamber by acombusting a by-propellant 7 which comprises igniting a rubbery spongehaving cells and an intercellular matrix of cured solid polymersubstance and propellant fuel in said matrix mixed with the curedpolymeric material, said cells containing a liquid oxidizing agentreactive with the fuel and the cured polymeric substance on ignition,said fuel and oxidizing agent composing at least 7.5 wt.% of theby-propellant.

10. A rocket propellant of blown and cured rubbery sponge having cellsand'intercellular matrix of cured rubbery polymeric substance containingadmixed therein a solid fuel, said cells containing a liquid oxidizingagent reactive with the solid fuel on ignition to generate combustiongas for propulsion, said oxidizing agent and said fuel composing aboutto wt.% of the propellant.

1. A rocket propellant which comprises a rubbery sponge having cells andan intercellular matrix of a solid rubbery polymeric substance havingcompounded therein a fuel, said polymeric substance being an elastomerblown into a sponge and cured, and said cells being filled with a liquidoxidizing agent reactive with the cured matrix and the fuel compoundedtherein on ignition to generate combustion gas for imparting propulsion,said oxidizing agent and fuel composing at least 75 wt.% of thepropellant.
 2. A rocket propellant according to claim 1, wherein theliquid oxidizing agent is thickened to a semi-liquid.
 3. A rocketpropellant according to claim 1, wherein the elastomer is a copolymer ofa mono-olefin and a diolefin.
 4. A rocket propellant according to claim1, wherein said fuel compounded with the rubbery polymeric substance isa powdered metal selected from Groups I to IV of the Periodic Chart ofElements.
 5. A rocket propellant according to claim 1, wherein therubbery polymeric substance is cured butyl rubber.
 6. A rocketpropellant according to claim 1, in which the rubbery polymericsubstance contains halogen.
 7. The method for preparing a rocketpropellant which comprises admixing a curable rubbery polymer with apowdered fuel, blowing agent, and curing agent, heating the resultingmixture at elevated temperature until a cured rubbery sponge havingcells and an intercellular matrix of the cured rubbery polymer andpowdered fuel mixed with the polymer in the matrix is obtained andfilling the cells of said sponge with a liquid oxidizing agent reactivewith the cured polymer and the fuel compounded therein on ignition, saidoxidizing agent and said fuel composing at least 75 wt.% of thepropellant.
 8. A method for preparing a hybrid rocket propellant whichcomprises admixing 100 parts by weight of a curable rubbery hydrocarbonpolymer with about 20 to 300 parts by weight of fuel, about 5 to 30parts by weight of a blowing agent and 1 to 20 parts by weight of acuring agent, heating the resulting mixture to an elevated temperatureuntil the mixture forms a cured open cell rubbery sponge andsubstantially filling the cells in said sponge with a liquid oxidizingagent reactive with the fuel, said oxidizing agent and said fuelcomprising at least 75 wt.% of the propellant.
 9. A METHOD FORDEVELOPING THRUST IN A ROCKET COMBUSTION CHAMBER BY COMBUSTING ABY-PROPELLANT WHICH COMPRISES IGNITING A RUBBERY SPONGE HAVING CELLS ANDAN INTERCELLULAR MATRIX OF CURED SOLID POLYMERIC SUBSTANCE ANDPROPELLENT FUEL IN SAID MATRIX MIXED WITH THE CURED POLYMERIC MATERIAL,SAID
 9. A method for developing thrust in rocket combustion chamber by acombusting a by-propellant which comprises igniting a rubbery spongehaving cells and an intercellular matrix of cured solid polymersubstance and propellant fuel in said matrix mixed with the curedpolymeric material, said cells containing a liquid oxidizing agentreactive with the fuel and the cured polymeric substance on ignition,said fuel and oxidizing agent composing at least 75 wt.% of theby-propellant.